A glen is a valley, typically one that is long, deep, and often glacially U-shaped, or one with a watercourse running through it. Whittow[1] defines it as a "Scottish term for a deep valley in the Highlands" that is "narrower than a strath."

The word is Goidelic: gleann in Scottish and Irish Gaelic, glion in Manx. In Manx, glan is also to be found meaning glen. It is cognate with Welshglyn. The word is sometimes found in tautological placenames where its meaning was opaque to a new linguistic community, an example perhaps being Glendale (literally "valley valley") which is a combination of Norse "dale" and Gaelic "glen".

As the name of a river, it is thought to derive from the Irish word glan meaning clean, or the Welsh word gleindid meaning purity. An example is the Glens of Antrim in Northern Ireland where nine glens radiate out from the Antrim plateau to the sea along the coast between Ballycastle and Larne.

In the Finger Lakes Region of New York State, the southern ends of Seneca Lake and Cayuga Lake in particular are etched with glens, although in this region the term "glen" refers most frequently to a narrow gorge, as opposed to a wider valley or strath. The steep hills surrounding these lakes are filled with loose shale from glacial moraines. This material has eroded over the past 10,000 years to produce rocky glens (e.g., Watkins Glen and Treman State Parks) and waterfalls (e.g., Taughannock Falls) as rainfall has descended toward the lakes below.